124

FLUIDITY AND PLASTICITY

ent value depending upon whether the oxygen was in a carbonyl
group, hydroxyl, ether, et cetera. We will now attempt to show
that this was necessary so long as viscosities formed the basis of
comparison, but it was not an evidence of constitutive influence,
and in comparing fluidities only one value for oxygen is obtained
irrespective of the manner in which it is combined, and yet we
have seen that satisfactory association factors are obtained.
Let AB and A'B* in Fig. 47 represent two fluidity curves, parallel
to each other and therefore presumably representing members
of the same class of substances, and let a third fluidity curve
CD be at an angle to the other two to represent a substance in
another class. Since we have elected to compare absolute

temperatures at a fluidity of 200, this
amounts to comparing the intercepts
of the curves on the line AD, whose
equation is <p = 200. The corre-
sponding viscosity curves obtained
by taking the reciprocal values of
the above fluidities and multiplying
by 10,000 are represented by a&,
a'6', and cd; and ad is of course the
reciprocal of the line AD. But the
point a and the point where the
curve a'V crosses ad are points of
equal slope on the viscosity curves, hence within a given class
it makes no great difference whether we compare temperatures
corresponding to a given fluidity or temperatures corresponding
to a given slope on the viscosity curves. The latter is exactly
the method of comparison which Thorpe and Rodger found
very advantageous. But between different classes they found
difficulties which they attributed to constitutive influences. But
their difficulty is now easily explained, for d is the true reciprocal
of the point D which we believe should be used in the com-
parison; on the other hand, they selected the point e, which has
the same slope as the point a, and for this choice we think that
there is not adequate reason.

It has been customary to assume that different kinds of
groupings should have special values assigned to them, and
particularly important among these was the "ring grouping.77

PIG. 47.—Diagram illustrating
the relationship between viscos-
ity- and fluidity-temperature
curves.